New Constraints on Axionlike Particles with the NEON Detector at a Nuclear Reactor

We report new constraints on axionlike particles (ALPs) using data from the NEON experiment, which features 16.7 kg of NaI(Tl) target located 23.7 m from a 2.8 GW thermal power nuclear reactor. Analyzing a total exposure of 3063 kg · day, with 1596 kg · day during reactor-on and 1467 kg · day during reactor-off periods, we compared energy spectra to search for ALP-induced signals. No significant signal was observed, enabling us to set exclusion limits at the 95% confidence level. These limits probe previously unexplored regions of the ALP parameter space, particularly for axion masses (m_𝑎) near 1 MeV/c². For ALP-photon coupling (𝑔_𝑎⁢𝛾), limits reach as low as 6.24×10⁻⁶ GeV⁻¹ at m_𝑎=3.0 MeV/c², while for ALP-electron coupling (𝑔_𝑎⁢𝑒), limits reach 4.95×10⁻⁸ at m_𝑎=1.02 MeV/c². This Letter demonstrates the potential for future reactor experiments to probe unexplored ALP parameter space.. Authors: Park, Byung Ju [IBS School; University of Science and Technology (UST); Institute for Basic Science (IBS)] (ORCID:0009000839071371); Choi, Jae Jin [Seoul National University; Institute for Basic Science (IBS)]; Jeon, Eunju [Institute for Basic Science (IBS); IBS School; University of Science and Technology (UST)] (ORCID:0000000159428907); Kim, Jinyu [Korea Atomic Energy Research Institute (KAERI)] (ORCID:0009000317812216); Kim, Kyungwon [Institute for Basic Science (IBS)] (ORCID:0000000176399511). DOE Contract: 89243024CSC000002; AC02-07CH11359; SC0010813. Subjects: 72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS; axions; nuclear reactors; particle dark matter